Termolecular Ion–Molecule Reactions in Titan's Atmosphere, I: Principal Ions with Principal Neutrals

Abstract

The FA-SIFT experiment at Canterbury was used to determine some of the principal termolecular ion–molecule reactions that occur in the atmosphere of Titan, Saturn's satellite. The experiments were performed using both a pure nitrogen bath gas and a pure helium bath gas. The reactions of the ions N +, N 2 +, N 3 +, N 4 +, HCNH +, c-C 3H 3 +, and H 3O + with molecular nitrogen were studied in a nitrogen bath gas at room temperature. Only two of these ions, N + and N 2 +, were observed to react with N 2, forming N 3 + and N 4 +, respectively. A second set of experiments was performed that measured the reactivity of ions in Titan's atmosphere that do not undergo rapid bimolecular reactions with other species. The reactions of these so-called “terminal” ions (HCNH +, c-C 3H 3 +, and H 3O +) with the trace neutral constituents of Titan (CH 4, C 2H 2, and C 2H 4) were also examined using both helium and nitrogen as the bath gas. The only reactions observed to occur were the termolecular reactions of HCNH + and H 3O + with C 2H 2 and C 2H 4. A third set of reactions was studied. These were the reactions of N 3 + and N 4 + with the species CH 4, C 2H 2, C 2H 4, HCN, and C 4H 8. All of the reactions in this third set have bimolecular ion–molecule kinetics, and they are compared in the text with the similar reactions of N + and N 2 + with these reagents. Finally, a projection is made using a model temperature and pressure profile of Titan's atmosphere to appraise the relative importance of N + vs N 3 + reactions and N 2 + vs N 4 + reactions.